An electronic overload relay detects a load current of a motor by a current detecting device (such as a CT), and if a detected load current exceeds a set value, the electronic overload relay flows operating current to a polarized electromagnet from a current detection circuit, and performs a trip operation for opening and closing a control circuit contact.
By the trip operation, a normally-open contact (contact a) is closed to turn on an indicator lamp, and a normally-closed contact (contact b) is opened to release excitation of an electromagnet of an electromagnetic contactor of a load circuit of a motor, thereby blocking the load circuit to prevent an accident such as burnout of the motor. After the trip operation, to restart the motor, it is necessary to perform a reset operation for returning the overload relay to a state before the trip operation (a state where the normally-open contact is opened and the normally-closed contact is closed).
The reset operation includes a manual reset operation that is performed by operating a reset bar, and an automatic reset operation that is performed by operating a polarized electromagnet using operating current output from a current detection circuit after a predetermined time is elapsed after the trip operation. This reset operation has to be performed after the cause of the overload of the load circuit of the motor is eliminated.
The overload relay needs a trip free function capable of performing a trip operation without any problem when an overload of the motor (load) is detected by a current detecting device even when an electric wire hits the reset bar for some sort of reason and the reset operation is performed, a manual/automatic switching function of the reset operation, and a function for prohibiting the reset operation for a predetermined time after the trip to prohibit the motor from restarting before the motor is cooled or the motor is recovered from its abnormal condition.
As a conventional overload relay having these three functions, there is an overload relay including a polarized electromagnet having a permanent magnet and a coil in a magnetic circuit, in which an armature is attracted and held in a reset position against a spring force by a magnetic field of the permanent magnet, and a magnetic field in a direction opposite to the permanent magnet is generated when an overload is detected, a contact mechanism that is operated in association with an armature, and a reset bar that returns the released armature to the reset position. This overload relay includes an inversion mechanism that is alternately inverted to a reset side and a trip side when crossing a dead center of a spring function, thereby switching the contact mechanism, the inversion mechanism is pushed by the released armature to invert the inversion mechanism to the trip side from the reset side, the inversion mechanism that is inverted to the trip side is pushed by the reset bar toward the reset side, and when the armature is released when an overload is detected, the coil is energized such that a magnetic field is generated in the same direction as the permanent magnet after a predetermined time is elapsed, the released armature is returned to the reset position and then, the reset bar is pushed, thereby inverting the inversion mechanism to the reset side (for example, see Patent Document 1).
According to this overload relay, the reset bar can be locked in its pushed-in state, and when the inversion mechanism is pushed toward the trip side by the armature in the pushed-in state, the inversion mechanism is prevented from inverting by the reset bar before crossing the dead center so that the automatic resetting can be performed.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-022203